Conventional drilling of well-bores to find and produce hydrocarbon deposits, entails rotation of a drill bit at the end of a string of drill pipe while pumping a fluid, commonly referred to as drilling mud, down through the string of drill pipe, through the bit, then up through the annulus back to the surface. The drilling mud is used to transport the solid particulates cut from the earth back to the surface, while incidentally cooling the drill bit, and in some cases driving a motor to rotate the bit. Another important function of the drilling mud is to control bottom hole pressure, and for this reason, the specific gravity of the mud is typically increased by special high density, fine particulate additives. The drilling mud is therefore comprised primarily of water in which a very fine particulate matter is dispersed and maintained in suspension. The materials added to the drilling mud are frequently deleterious to the environment, and because of the colloidal nature of the suspension, tend to be very slow drying.
When the drilling fluids are returned to the surface, the cuttings produced by the bit are entrained in the mud and typically include particulates having a wide range of sizes. The drilling mud bearing the cuttings is normally returned by gravity to a shale shaker which screens larger particles from the fluid and then is passed through a series of metal working tanks placed above ground level before being pumped back through the drill string to the drill bit. The working pits are used to condition the mud for return to the bore hole. A major portion of the cuttings which are not screened by the shale shaker are allowed to settle to the bottom of the first pit. This process is facilitated by a series of mechanical stirring devices which attempt to maintain the very fine particles in the drilling fluid sufficiently stirred to keep the particles in suspension while simultaneously facilitating the settlement by gravity of the larger cuttings from the borehole. As many working pits as required are used for separation and to maintain a sufficient volumetric reserve of drilling fluids for the drilling operations. Various materials are added to the muds such as high specific gravity particulates, chemicals or the like, typically added to the last working pit or suction pit prior to the mud pumps. It is also frequently necessary to add water to maintain a mud having an appropriate viscosity.
As part of the preparation for drilling the well, a reserve pit is normally formed in the surface of the earth using a bulldozer to scrape the earth down to the desired depth with the dirt piled at one end so the dirt can be conveniently used to recover the pit when the well is completed. During the drilling operations, the particulate materials screened by the shale shaker are transferred to the reserve mud pit, and the cuttings which settle from the mud in the working pits are periodically flushed out during periods when mud circulation is interpreted during normal operations. The material flushed from these pits is also directed to the earthen mud pit, as well as any drilling fluid which, for one reason or another, is deemed inappropriate for further drilling operations. Ultimately, all drilling fluids used in the operation are typically placed in the mud pit. Since in the process of finding the hydrocarbons, a certain amount of hydrocarbons are invariably entrained in the mud, and ultimately deposited in the mud pit, as well as excess cement from the well completion process, the mud pit commonly includes hazardous waste material which must be dealt with by either providing a pit lined with a long-lasting impervious material, or the mud must be pumped from the working pits and/or reserve pit into transports and then hauled to a suitable waste disposal site.
The mechanical stirring devices used to agitate the drilling fluid in the working pits require substantial power to operate, are somewhat complex and expensive to construct, and require substantial maintenance. Additionally, it is very difficult for these devices to provide uniform agitation within the pits and is common for dead spots to exist which result in settling and accumulating the fine particles from the drilling fluid. The result is that an excessive preparation of the expensive fine particulate additives used to enhance specific gravity are lost as sediments in the bottoms of the working tanks and are flushed to the earthen pit and expensive additives must be replaced to maintain the desired mud weight.
When the drilling fluid is ultimately disposed of, the historical approach was to leave the drilling fluid in the open pit to dry by natural evaporation. This typically took a substantial period of time during which there was danger of cattle or other animals wandering into the pit and perishing. Eventually, a bulldozer was used to push the dirt originally removed from the pit over the dried material and return the surface as nearly as possible to its original condition. If this process was attempted before the drilling mud had adequately dried, the bulldozer was at risk of becoming mired in the wet drilling mud. Sometimes the dry soil displaced excessively wet drilling mud to the surface causing significant problems. In more recent times, environmental concerns have resulted in drilling pits being lined with zippered or otherwise seamed plastic sheets to prevent migration of the oils and chemicals into the ground water. This has further complicated the covering process because of the danger of the bulldozer puncturing the lining. In some instances, environmental concerns have resulted in the material being accumulated and trucked away in the fluid or semi-fluid state to an approved disposal site.